1. Field of the Invention
This invention relates to a lithographic alignment system particularly one usable in a system employing X-ray or other beams for printing replicas of patterns or masks on photo sensitive materials contained on semiconductor substrates such as silicon wafers, wherein the inert gas between the X-ray source and mask and the wafer fabrication process gas between the mask and wafer are controlled in terms of purity attenuation, oxygen content and leakage losses, and in pressurization levels affecting the mask-to-wafer gap.
2. Description of the Prior Art
U.S. Pat. No. 4,185,202 discloses as prior art an X-ray lithography system involving an X-ray source, an X-ray transmission chamber 14 and an imaging or exposing chamber 18 into which a prealigned mask-wafer fixture is moved into position under the source at the bottom of the exposing chamber for each exposure. The transmission chamber is maintained in a vacuum at a pressure less than about 10.sup.-6 torr while helium is present in the imaging or exposure chamber or a 10.sup.-2 torr vacuum provided which limits the throughout capabilities of the system. The improvement in the '202 patent is directed to having a movable (expandible) exposure chamber 40 with a side wall 46 which is lowered to engage the mask-wafer fixture. Helium is flowed into chamber 40 for each exposure at a rate of 10 liters/minute through passageway 42. A separate atmosphere of inert gas e.g. nitrogen at a rate of 2.9 liter/minute at a pressure of 5-50 psi is provided into the fixture and the zone between the mask underside and the wafer. Flushing of the zone is initially started while the fixture is still being loaded remote from the exposure chamber. The '202 patent also mentions that it may be advantageous to maintain the pressure in the exposure chamber greater than in the mask-to-wafer zone to prevent gaseous constituents in the latter being introduced to the former and contemplates addition of a small amount of inhibiting oxygen in the mask-to-wafer zone.
In the prior art the respective gases are both allowed to escape to the atmosphere in a relatively uncontrolled manner, typically the helium leaking profusely near the mask edge and exiting from the junction of the chamber and a bottom frame, and the process gas being injected in the vicinity of the wafer edge, in an attempt to maintain the proper gas environment and allowed to exit between the chuck and bottom frame.